[Rd] sub and gsub treat \\ incorrectly (PR#13454)

[Prof Brian Ripley]

On Tue, 20 Jan 2009, Andriy Miranskyy wrote:

> Thank you, William! This makes things clearer.
>
> I am trying to create a tiny converter of free text to Tex format. In
> order to do that I need to replace all "_" with "\_" and all "&"
> with "\&". Could you please tell me, is there a way of doing it using
> gsub?

Yes.  gsub("([_&])", "\\\\\\1", x) -- but you will also need to escape 
$ and %.  There are examples in the experimental Rd2tex function in 
the development version of R.

Please stop misusing the bug repository to ask questions (and to 
report your non-reading of the documentation: this is in ?regex 
referenced from ?sub, 'explicitly', pace Bill).

>
> Regards,
> Andriy
>
> Monday, January 19, 2009, 6:24:56 PM, you wrote:
>
>>> -----Original Message-----
>>> From: r-devel-bounces at r-project.org
>>> [mailto:r-devel-bounces at r-project.org] On Behalf Of amiransk at uwo.ca
>>> Sent: Monday, January 19, 2009 10:25 AM
>>> To: r-devel at stat.math.ethz.ch
>>> Cc: R-bugs at r-project.org
>>> Subject: [Rd] sub and gsub treat \\ incorrectly (PR#13454)
>>>
>>> Sub and gsub treat \\ replacement pattern incorrectly
>>>
>>> I expect
>>>   sub("a","\\", "a", perl=T)
>>> to produce
>>>   [1] "\"
>>> instead it generates
>>>   [1] ""
>>>
>>> On the other hand, if I run
>>>   sub("a","\\\\", "a", perl=T)
>>> it correctly outputs
>>>   [1] "\\"
>
>> The replacement pattern may include \\digit, which means
>> to put the digit'th parenthesized subexpression into the
>> replacement.  E.g.
>>   > sub("([[:alpha:]]+) +([[:alpha:]]+)", "\\2 \\1", "One two three
>> four five")
>>    [1] "two One three four five"
>>   > gsub("([[:alpha:]]+) +([[:alpha:]]+)", "\\2 \\1", "One two three
>> four five")
>>    [1] "two One four three five"
>> To support this without ambiguity or surprises, \\ is expected
>> to be followed by a digit (or L or U when perl=TRUE).
>
>> When fixed=TRUE then there is no possibility of a parenthesized
>> subexpression so \\2 is taken literally.
>
>> help(gsub) is not explicit about this behavior.
>
>> Because I initially made the same mistake, when I wrote the S+
>> versions of gsub and sub I included a warning when the replacement
>> included a \\ not followed by a digit:
>
>  >> gsub("([[:alpha:]]+) +([[:alpha:]]+)", "\\ \\", "One two three four
>> five")
>>   [1] "    five"
>>   Warning messages:
>>     backslash in replacement argument of substituteString(fixed=F) is
>> not
>>           followed by backslash or digit, hence backslash is omitted in:
>> substit\
>>           uteString(pattern = pattern, replacement = replacement, x = x,
>> extended ....
>
>>> The same issue applies to gsub.
>>>
>>> --please do not edit the information below--
>>>
>>> Version:
>>>  platform = i386-pc-mingw32
>>>  arch = i386
>>>  os = mingw32
>>>  system = i386, mingw32
>>>  status =
>>>  major = 2
>>>  minor = 8.1
>>>  year = 2008
>>>  month = 12
>>>  day = 22
>>>  svn rev = 47281
>>>  language = R
>>>  version.string = R version 2.8.1 (2008-12-22)
>>>
>>> Windows XP (build 2600) Service Pack 2
>>>
>>> Locale:
>>> LC_COLLATE=English_United States.1252;LC_CTYPE=English_United
>>> States.1252;LC_MONETARY=English_United
>>> States.1252;LC_NUMERIC=C;LC_TIME=English_United States.1252
>>>
>>> Search Path:
>>>  .GlobalEnv, package:stats, package:graphics,
>>> package:grDevices, package:utils, package:datasets,
>>> package:methods, Autoloads, package:base
>>>
>>> --
>>> Sincerely,
>>>  Andriy
>>>
>>> ______________________________________________
>>> R-devel at r-project.org mailing list
>>> https://stat.ethz.ch/mailman/listinfo/r-devel
>>>
>
> ______________________________________________
> R-devel at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-devel
>

-- 
Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272866 (PA)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595